490 Causes and Course of Organic Evolution 



marsupials and marsupials became increasingly abundant, 

 enlarged in size, and varied in generic detail, till in mid-tertiary 

 times they had spread over North and South America, Europe, 

 and possibly southern Asia and East Africa. 



Now, though the very name of the group is derived from 

 the presence in some of a marsupium or pouch for sheltering 

 the young, all morphological and palseontological e\ddence 

 indicates that such is an acquired, not a primitive, character 

 of the group.- For of the two subdivisions of the family now 

 living, viz., the Polyprotodontia and Diprotodontia, the former 

 is the more ancient and structurally is the most primitive. 

 It is also the group that still extends from North and South 

 America to Australia. Thus in the Jurassic age Spalacotherium, 

 Amphilestes, and other genera represent a series of small crea- 

 tures varying from a mouse to a rat in size, and which from 

 the structure of the jaw and teeth conform to the above group. 

 In more recent strata additional genera occur along with ex- 

 amples of Diprotodontia that seem to have evolved contem- 

 poraneously with groups of carnivorous and insectivorous 

 mammals to which reference will later be made. 



But, in thinking of a possible connection between such 

 Jurassic or even older forms and the urodele Batrachia, the 

 most fundamental question that can concern us is the devel- 

 opment of the embryo and its nutritive connection with the 

 parent. While the great majority of the urodeles and all 

 of the reptiles are oviparous, mention has already been made 

 of viviparous salamanders. We may therefore compare the 

 maternal and embryonic relations in the last with the con- 

 ditions seen in marsupials. In the simplest urodeles like Siren 

 and Proteus the allantoic bladder (urinary bladder) is a narrow 

 elongated sac that opens on the ventral side of the cloaca, 

 but in higher groups it becomes swollen above, or even the 

 swelling may become bilobed. In marsupials the allantoic 

 bladder is comparatively small as in the last, and never expands 

 in any part to form a nutritive and respiratory sac for the 

 embryo, except in the genus Perameles where a true allantoic 

 I)lacenta is formed. This, amongst other characters that will 

 later be referred to, indicates that it is from one of the primi- 

 tive polyprotodonts that most, though not all, characters have 

 evolved. 



